Many silicon-based micro-sensors use so-called MEMS (microelectromechanical systems) technology to achieve low cost and high performance. One such a device is a MEMS pressure sensor, which is comprised of a small, thin silicon diaphragm onto which a piezoresistive circuit, normally a Wheatstone bridge, is formed. Diaphragm stresses caused by pressure applied to the diaphragm change the resistance values of the piezoresistors in the bridge circuit. An electronic circuit detects the resistance changes of the piezoresistive bridge and outputs an electrical signal representative of the applied pressure.
FIG. 1A is a cross-sectional view of a prior art differential pressure sensor 100, so named because it provides an output signal representative of the pressure difference between the top pressure and the bottom pressure on the diaphragm 122 of FIG. 1B of the differential pressure sensing element 102 shown in FIG. 1B. FIG. 1B is a cross-sectional diagram of a differential pressure sensing element 102 mounted inside the housing depicted in FIG. 1A.
In FIG. 1A, the pressure sensor 100 is comprised of a housing 104 that encloses a MEMS pressure sensing element 102 and an application-specific integrated circuit (ASIC) 106. One fluid pressure from liquids or gases is applied to the bottom of the diaphragm of the MEMS pressure sensing element through a pressure port 108 formed into the housing 104. The other fluid pressure from gases through the cover 107 is applied to the top of the gel 124 which passes the pressure to the top of the diaphragm of the MEMS pressure sensing element (or silicon die) 102. The MEMS pressure sensing element 102 is electrically connected to ASIC 106 by conductive wires 103, well-known in the prior art and which provide electrical connections between the ASIC 106 and the pressure sensing element 102. Conductive wires also connect the ASIC 106 to the leadframes 105 for the input and output voltages.
As stated above, FIG. 1B is a cross-sectional diagram of a prior art MEMS pressure sensing element 102. The MEMS pressure sensing element 102 is a differential pressure sensing element comprised of a thin silicon die 110. The die 110 is formed from silicon. A piezoresistive Wheatstone bridge circuit 112 is formed in the die 110 and located near the edge of a thin diaphragm region 114.
The die 110 sits atop a pedestal 116, which is in turn attached to the housing 104 by an adhesive 120. Fluid that flows in the port 108 applies pressure to the bottom of diaphragm 122 formed by the placement of the die 110 over the port 108. The other fluid flows to the top of gel 124 and pressurizes the top of diaphragm 122. Arrows 123 represent pressure applied to the top and bottom of the diaphragm. A differential pressure or a difference between the pressure 123 applied downwardly and the pressure 123 applied upwardly causes the diaphragm 122 to deflect. The deflection caused by the pressure difference causes the piezoresistors in the bridge circuit 112 to change their physical dimensions which in turn changes their resistive values. The MEMS pressure sensing element 102 shown in FIG. 1B can be seen in FIG. 1A embedded in a conventional gel 124, an intended function of which is to protect the sensing element 102.